1. Field of the Invention
The present invention relates to an image forming or producing apparatus for forming an image on a sheet of recording paper through an electrophotographic process.
2. Description of the Related Art
With recent increase in the processing speed of personal computers, workstations, and the like, it has been desired to increase the operation speed of image forming apparatuses using the electrophotographic process. FIG. 9 schematically shows a construction of a conventional image forming apparatus. It comprises process cartridge 4 enclosing photosensitive drum 1, electrifier 2 and developing portion 3, exposure optical system 5, transfer roller 6, deelectrifier 7, fixing portion 8, paper cassette 9, paper supply 10, paper exit tray 11 and drive motor 12. Electrifier 2 opposing the surface of photosensitive drum 1 comprises wire 13 formed of tungsten for example and mesh-structured grid plate 14 and electrifies the surface of drum 1 along the rotational direction (of arrow A) of the drum. Exposure optical system 5 forms an electrostatic latent image by optical modulation of image data and comprises constituents such as semiconductor lasers, lenses, and a polygon mirror for reflecting semiconductor laser beams.
In such an image forming apparatus configured as above, the character printing operation can be conducted at a higher speed by increasing the rotary speed of the polygonal mirror included in the exposure optical system of the apparatus.
However, the rotary speed of the mirror has already been nearly maximized in the prior art. Consequently, as an alternative countermeasure to achieve the high-speed operation, there has been employed a method in which a plurality of semiconductor easer units are disposed on a surface of a photosensitive drum to produce an electrostatic latent image so as to record a plurality of lines by the plural semiconductor lasers at the same time.
Next, description will be given of the configuration and operation of a conventional image forming apparatus including a plurality of semiconductor lasers. FIG. 5 is a diagram showing the configuration of an exposure optical system of an image forming apparatus including a plurality of semiconductor laser units, to which the present invention is applicable. In FIG. 5, the exposure optical system includes a first semiconductor laser 21 and a second semiconductor laser 22 arranged such that laser beam emitting directions thereof are orthogonal to each other. Additionally, the first and second lasers 21 and 22 are respectively connected to laser drivers 30 and 31. The drivers 30 and 31 modulate image data sent from an image processing section, not shown, and thereby respectively cause the lasers 21 and 22 to emit laser beams.
At a position in the laser beam emitting direction of the first laser 21, there is disposed a collimator lens 23 to collimate the laser beam. Similarly arranged at a position in the laser beam emitting direction of the second laser 22 is a collimator lens 24 for the same purpose. Furthermore, a beam splitter 25 is provided on the beam emitting side of the lenses 23 and 24. The beam splitter 25 orthogonally deflects the laser beam from the first laser 21 and allows the laser beam from the second laser 22 to pass therethrough and introduces the beams to a surface of a polygonal mirror 26. The mirror 26 is disposed about an axis of rotation of a polygonal mirror motor, not shown, rotating at a high speed to achieve a scanning operation by use of the laser beams from the lasers 21 and 22 in a direction toward the side of a photo-sensitive drum 1. Between the polygonal mirror 26 and the photosensitive drum 1, there are disposed an f.theta. lens 27 and a reflection mirror 28, the lens 27 decreasing the diameter of the laser beam reflected on the surface of the mirror 26 into a predetermined size. Moreover, arranged between the lens 27 and the mirror 28 is a sensor 29 to sense a reference position of the laser beam for the scanning operation.
In the image forming apparatus, using two laser beams radiated respectively from the first and second semiconductor lasers 21 and 22, it is possible to simultaneously record image data of two lines on the surface of the photosensitive drum 1 in a primary scanning direction B of the photosensitive drum 1.
FIG. 7 is a control block diagram of the conventional image forming apparatus including a plurality of semiconductor lasers. The configuration and operation of a control system of the image forming apparatus will be described by referring to FIG. 7.
In FIG. 7, image data generating means 34 generates image data (bit map data) according to image information from a host computer, not shown. Storage means 35 stores therein the image data generated by the image data generating means 34. First buffer means 38 and second buffer means 39 acquire from the storage means 35 image data items respectively related to first exposing means 36 and second exposing means 37, which will be described later, and then electrically amplify the data items to resultantly output amplified data items therefrom. First and second image processing means 40 and 41 conduct various kinds of image processing for the image data items respectively from the first and second buffer means 38 and 39 and output obtained data items respectively to the first and second exposing means 36 and 37. As shown in FIG. 5, the first and second exposing means 36 and 37 respectively include the first and second semiconductor lasers 21 and 22 to emit laser beams according to signals respectively from the first and second image processing means 40 and 41.
Timing control means 42 produces a print reference signal according to a scan position reference signal of the laser beam from each of the first and second exposing means 36 and 37. According to the print reference signal from the timing control means 42, output control means 43 controls the operation to sequentially output the image data from the storage means 35 to the first and second buffer means 38 and 39.
Next, operation of the image forming apparatus will be described.
In FIG. 7, the image data generating means 34 generates image data (bit map data) according to the image information from the host computer (not shown) and then stores the data in the storage means 35. In response to the print reference signal from the timing control means 42, the output control means 43 controls the storage means 35 to sequentially output the image data of raster 1 to the second buffer means 39 and the image data of raster 2 to the first buffer means 38.
While the image data of raster 1 is electrically amplified in the second buffer means 39 to be inputted to the second image processing means 41, the image data of raster 2 is electrically amplified in the first buffer means 38 to be inputted to the first image processing means 40. The first and second image processing means 40 and 41 correct variations in the printout quality due to fluctuation in the operational environments of the image forming apparatus and in the characteristics unique to the apparatus. For this purpose, the variations in the operation environment of the image forming apparatus and in the characteristics unique to the apparatus are sensed by various sensors, not shown, to correct the input image data according to the sensed data. The corrected image data items are then respectively delivered to the first and second exposing means 36 and 37.
Referring again to FIG. 5, the first and second exposing means 36 and 37 optically modulate the image data items by the first and second semiconductor lasers 21 and 22, respectively. The resultant laser beams are respectively converted through the collimator lenses 23 and 24 into parallel rays or beams. Thereafter, the laser beam from the first laser 21 is deflected 90.degree. by the beam splitter 25 and the laser beam from the second laser 22 is passed through the beam splitter 25. The beams are reflected on a surface of the polygonal mirror 26 for the scanning operation at a predetermined angle in accordance with the rotary speed of the mirror 26.
Each of the laser beams which are sent from the first and second lasers 21 and 22 and which are reflected on the mirror 26 enters the lens 27 such that the diameter of the beam is reduced to a predetermined beam diameter and is then reflected on the reflection mirror 28 such that the beam scans the surface of the photosensitive drum 1 in a direction of arrow B.
The timing control means 42 receives a sense signal from the sensor 29 to generate a print reference signal. The print reference signal is acquired at a point of time when a predetermined period of time lapses after the sensor 29 detects the laser beam. The output control means 43 starts an operation to read image data from the storage means 35 in accordance with the print reference signal from the timing control means 42.
The photosensitive drum 1 rotates in a direction of arrow A at a fixed speed Vp (mm/sec). In consequence, when one scanning operation is completed in the direction of arrow B, the next scanning operation is initiated in the direction of arrow B at a position where the drum 1 has rotated a predetermined angle in the direction A.
FIG. 6 is a diagram showing an image forming operation in the conventional image forming apparatus, which shows a case correctly formed without unfavorable positional shifts or displacements of images formed on the drum. As can be seen from FIG. 6, a beam spot 33 of the second semiconductor laser 22 and a beam spot 32 of the first semiconductor laser 21 respectively conduct scanning operations for rasters 1 and 2 at the same time. Thereafter, the scanning operation is sequentially conducted for two rasters at a time, for example, rasters 3 and 4 such that image data is recorded as an electro-static latent image on the photosensitive drum 1 at a print speed which is twice the ordinary print speed.
However, in the exposure optical system of the conventional image forming apparatus, the image forming positions of the beam spots 32 and 33 on the surface of the drum 1 respectively of the lasers 21 and 22 may be different or shifted from each other in the primary scanning direction (arrow B).
FIG. 8 is an image forming diagram showing a case in which the image forming positions of the exposure optical system vary from each other in the conventional image forming apparatus. In this case, the output timing of image data from the storage means 35 is determined in response to the print reference signal from the timing control means 42 as described above. Namely, the same output timing applies to the image data items respectively of the first and second exposing means. Consequently, the image of rasters 2 and 4 printed by the first exposing means 36 is positionally shifted from that of rasters 1 and 3 printed by the second exposing means 37, which leads to a problem of considerable deterioration in the quality of the printed image.
To solve this problem, the image forming positions respectively of the beam spots 32 and 33 respectively of the first and second lasers 21 and 22 are adjusted to prevent the mismatching or shift between the printed images in the conventional technology. In accordance with the prior art, the image forming apparatus including a plurality of semiconductor lasers has been an expensive apparatus and hence there are produced a relatively small number of such image forming apparatuses. However, recently, the position adjustment is also conducted for image forming apparatuses which are broadly used and of which the production number is large. In consequence, the production method in which the exposure optical system is adjusted for each apparatus causes a considerable decrease in productivity and soaring production costs. Namely, such adjustments cannot be actually carried out in practical cases.